The present invention relates to a system for controlling fuel injection in an automotive engine in dependency on pressure in cylinders of the engine. The pressure in the cylinders is used as a parameter representing the quantity of intake air, for deciding an air-fuel ratio of mixture.
In a known fuel injection system, a fuel injection quantity is calculated based on the intake air quantity which is detected by an airflow meter. However, the airflow meter is not accurate enough for obtaining an optimum air-fuel ratio. Thus, there has been proposed a system for accurately controlling the fuel injection where the quantity of fuel to be injected is calculated in dependency on the pressure of intake air in the cylinder. The system excels the system with the airflow meter in accuracy. However, when a piezoelectric sensor is used for detecting absolute pressure, accurate measurement is not obtained because of the drift of the sensor.
Japanese Patent Application Laid Open No. 60-47836 discloses a system for controlling air-fuel ratio, wherein the quantity of intake air is obtained from the difference P between a minimum pressure Pmin detected at the bottom dead center (BDC) and a pressure detected at the crank angle of 140 degrees. By using the relative pressure, the problem of the drift in the pressure is solved.
However, the intake valve of the cylinder having the pressure sensor is still open at the BDC so that the minimum pressure Pmin in the cylinder oscillates because of pulsation of intake air. Consequently, the pressure is not accurately detected.
In addition, in the control system, when an accelerator pedal of a vehicle is depressed for accelerating the vehicle, a lean spike or a rich spike of the air-fuel ratio is formed as a result of inappropriate control of fuel quantity.